Proceedings e report - Firenze University Press

SORPTION OF MOISTURE AND DIMENSIONAL CHANGE OF WOOD SPECIES
USED IN HISTORIC OBJECTS
Lukasz Bratasz*, Roman Kozlowski, Antonina Kozlowska, Bartosz Rachwal
Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences,
ul. Niezapominajek 8, 30-239 Kraków, Poland
Abstract
‘General’ moisture sorption and swelling/shrinkage patterns which would apply as a first approximation to any
wood species constituting cultural objects were proposed. They were obtained from the experimental data
measured for 21 historically important wood species used in the past for panel paintings and woodcarving.
Information on further wood species of interest to the wood conservation community can be added to the
database and used to constantly improve the general relationships.
1. Introduction
Uncontrolled variations of ambient temperature and relative humidity have been the principal agents
of deterioration for historic and artistic wooden objects. The variations in the environment induce
physical damage to the wood due to its hygroscopic nature and dimensional response to the moisture
sorption or desorption.
Over the centuries, a great number of wood species have been used for constructional, utilitarian and
decorative purposes with each geographic region, historic period and application field favouring
specific woods for reasons of availability, ease of processing or fashion. Therefore museum
collections or interior furnishings of historic buildings are mixture of various wood species having
different responses to climatic variations. Wood variability within species and due to aging adds an
additional dimension to the complexity of the problem. As a result, acquiring information on moisture
sorption and related dimensional response by wood species present in a particular collection or
building can be rarely possible due to high cost and long time necessary for the laboratory work, as
well as a difficulty in withdrawal of samples from cultural objects. By necessity, the assessment of
wood damage risk related to the fluctuating climate should be rather based on some general climate –
wood relationships which for most practical purposes could be applied to mixed wood collections or
furnishings.
Therefore, the main aim of this study was to evaluate water vapour sorption and dimensional response
for 21 historically important wood species used in the past for panel paintings and woodcarving and to
propose a ‘general’ moisture sorption and swelling/shrinkage patterns which would apply as a first
approximation to any wood species constituting cultural objects, as well as ranges of moisture uptake
and dimensional response behaviour which can be expected.
2. Materials and methods
Wood species selected for this study are listed in Table 1. The selection was based on the list of types
of woods used in the past for panel paintings and woodcarving published by Grosser and Geier [1] for
softwoods, and by Grosser and Grässle [2] for hardwoods. The specimens were obtained from defectfree
wooden boards seasoned for at least 3 years at room conditions at the restoration workshop of the
National Museum in Krakow, Poland.
Moisture adsorption and desorption isotherms were determined at 20 o C and for a full range of water
vapour relative pressures. The measurements were done gravimetrically with the use of a Sartorius
vacuum microbalance. Typically, a 0.05 g piece of wood was weighed and outgassed prior to a
measurement under a vacuum of a residual pressure less than 10 -3 mbar. The aim was to move air out
of the wood and to eliminate most of the species physisorbed during storage of the sample, especially
water. Vacuum was maintained until a constant weight was obtained, then subsequent portions of
* email: ncbratas@cyf-kr.edu.pl
Joseph Gril (edited by), Wood Science for Conservation of Cultural Heritage –Braga 2008: Proceedings of the International
Conference held by COST Action IE0601 (Braga - Portugal, 5-7 November 2008, ISBN 978-88-6453-157-1 (print)
ISBN 978-88-6453-165-6 (online) © 2010 Firenze University Press